People's perceptions of health risk form an important factor in many models of health behaviors, including those aiming to understand risk for HIV infection. At present, however, there is no recognized reliable and valid measure of perceived HIV risk, and a lack of research examining the roles of different aspects of risk perception. The aim of the proposed study is to develop a reliable and valid measure of perceived risk for HIV infection that incorporates different dimensions of perceived risk. Item Response Theory (IRT) will be employed to develop the new measure, initially in sample of drug users. IRT allows item characteristics to be assessed and reliable items to be selected to form a sensitive and precise scale. IRT will also be used to evaluate the psychometric properties of the new measure in a second high-risk group of Men who have Sex with Men (MSM) and Men who have Sex with Men and Women (MSMW). Cross-sectional data will be used to assess the relationship between health-risk behaviors and HIV risk perception (construct validity). The development of a reliable and valid measure of perceived risk of HIV infection, that incorporates different aspects of risk perception could inform both theories and interventions aiming to understand HIV risk behaviors.